Most modern systems have well defined operating states, or modes of operation and most of these systems use one or more clocks. Typical high performance applications are locked to a crystal oscillator operating between 1 MHz to 100 MHz, which helps to perform important internal operations of the chip with low jitter and least timing uncertainty. However, in many systems, particularly those that are portable, when these high performance operations are not active, the system is said to be in a low power state or low power mode, when power saving is much more critical for overall battery life. Some systems may even have more than two such operating modes or states. Since the logic power consumption is directly proportional to the clock frequency, most systems use less than 100 KHz for low power time keeping operations, when the device is operating in a low power mode. It is desirable to shut down all system functionality when the device is operating in a low power state. Power reduction benefits are higher if even the crystal oscillator generating the main system clock in MHz range can be shut down while still maintaining a low power kHz oscillator to keep time and to permit the system to resume from a sleep state. Crystal oscillators, utilizing crystal resonators, are commonly used to provide the MHz and kHz frequency clock signals when frequency stability is important. In most modern systems, a MHz oscillator commonly supports a high performance clock generator and in many systems, while many circuits can be powered down to reduce the power consumption of the circuit, long and PVT (process, voltage, temperature) dependent, widely varying startup times prohibit powering down the MHz crystal oscillator particularly, if the time spent in low power mode is relatively short. The kHz time keeping oscillator consumes low currents, but its low power (and hence limited gain) and high quality factor (Q) result in even longer startup times, which may not be acceptable in certain systems. Additionally, a 32.768 kHz frequency is typically used for the low power kHz oscillator, mainly due to the legacy reasons and thus the affordability of the crystal, however, some systems may prefer different frequencies. For example, while a 32.768 kHz frequency clock is convenient when a traditional legacy real-time system clock timer is needed, many modern systems don't need to generate this specific frequency, but still require a very accurate time-keeping circuit so that the system can wake up at a pre-calculated time, perform the required tasks and then return to a sleep state, without having to spend too much time, and hence power, synchronizing and starting up the MHz oscillator. In addition, some systems also require time keeping frequencies different from 32.768 kHz, in order to be able to better synchronize with other main logic circuits, which may not be running at an integer multiple of the 32.768 kHz clock. Some examples are 32 kHz, 50 kHz or 100 kHz.
In the prior art, when generating a high performance MHz clock and a low power time keeping kHz clock, two different crystal resonators are used, one for the high performance MHz clock and one for the low power kHz clock. The use of two separate resonators and their respective load tuning capacitors consumes a considerable amount of board space and increases the bill of material cost of the device. It is obvious that since the two different resonators at two different frequencies are used in these two different oscillators, their frequencies and phases may vary randomly over time, with respect to each other, further underscoring the synchronization difficulties in such systems between different clock domains.
Accordingly, what is needed in the art is a system and method for generating both a high performance clock signal and a low power clock signal which provides a reduction in area and in cost of the device (system), while providing increased flexibility in the circuit design utilizing the system providing the clock signals.